adjust text size:

Driveability Corner

Honda, like other makers, relies on cylinder deactivation to improve engine efficiency and fuel economy. Mark uses scan data to back up his understanding of its effect on throttle opening and load.

It’s a simple concept, cylinder disabling in variable displacement engines. Just disable a few cylinders and the remaining cylinders have to work harder. Working harder like a smaller engine requires a wider throttle opening. A wider throttle opening lowers intake manifold vacuum and therefore has less suction throttling loss and higher effective compression. Fewer cylinders equals better efficiency and fuel economy.

The Honda V6 variable cylinder management (VCM) system deactivates one-third (two) or half (three) of the cylinders, depending on load. With three cylinders deactivated and three carrying the load, the throttle has to open wider to accommodate the extra air needed by the working cylinders. The intake manifold vacuum will drop relative to the wider throttle opening. Take the time to go to Honda’s website, http://world.honda.com/automobile-technology/VCM/, for a complete explanation of the VCM operation.

Honda deactivates cylinders using a sliding pin that decouples the rocker arm. The intake and exhaust valves are decoupled and the cylinders are effectively sealed. The compression stroke has no intake and no exhaust; any air compressed is decompressed on the “power” stroke and pumping losses on the deactivated cylinders are eliminated.

Under the same load and rpm conditions, the engine will need the same air and fuel regardless of how many cylinders are pulling—minus the air and fuel saved from the reduced load from the drop in suction throttling. In general, the increased efficiency from the reduction in suction throttling loss is 5% to 8%—not huge but not bad, either.

With three of the cylinders deactivated, we can make the following logical assumptions:

•The mass airflow (MAF) will decrease in direct proportion to the increase in efficiency. Less load equals less air and fuel required.

•Fuel economy will increase.

•The ECO mode light on the dash will indicate VCM operation. (This is what the dealer told my daughter; the manual states only that the ECO mode light indicates efficient operation.)

Now might be a good time to point out that there is some controversy about the effectiveness of the VCM system. Do a Web search on the VCM and you’ll see that the system effectiveness is being questioned by some. We should be able to answer the efficiency question here by watching the scan data and seeing an overall decrease in MAF in direct proportion to the reduced load. Technically, it all makes perfect sense.

Now let’s look at the scan data of a 2012 Honda Pilot 3.5L V6 in action. Take a look at Fig. 1 on page 18. I’m driving the Pilot on a relatively level section of I-10. It’s difficult to find perfectly level roads outside of the midwest. You have to kind of look at the averages in the data to smooth out the road irregularities.

The top graph in Fig. 1 is vehicle speed. You can see the relatively long periods of steady-speed 65 mph and the corresponding steady rpm readings (third graph). The rpm spikes are where I hit the throttle for a short period to bring all six cylinders on line to watch the effect of the cylinder deactivation just after. You can see the corresponding throttle position sensor (TPS) spikes. Fig. 1 establishes a baseline for the driving condition.

Now let’s zoom in on a steady part of Fig. 2 on page 20, from 2190 to 2340 seconds, between two full-throttle events (time at the top of the graphs). Also, we’ll add in the MAF as the bottom graph. The first graph is vehicle speed—pretty rock-steady at 65 mph. The next graph is intake manifold vacuum (in red) overlaid with throttle position (green). The intake vacuum and throttle position are inversely proportional; as the throttle opens, the vacuum decreases. This is important to watch to filter out the effects of the varying throttle.

At the full-throttle events, vacuum dropped, as expected. Immediately after full throttle, vacuum spiked, as the throttle was reduced and rpm was still high, also as expected. Presumably, all cylinders were firing directly after the full-throttle event, per Honda’s explanation. At 2215 seconds, the effect of the full-throttle event seemed to settle out. At 2217 seconds, the vacuum is increasing as MAF is decreasing—what I might expect on an efficient engine with no VCM system.

The only spot that looks like a definite VCM operation is at 2300 to 2310 seconds, where the throttle is dead steady and vacuum shows a substantial change from about 13.5 to about 8.5 in. The MAF changed from 26.9 to 24.13 gm/sec., a change of 2.77gm/sec., or 10%. Also, note the roughness in the intake manifold vacuum signal at this point, likely indicating fewer cylinders operating.

To get a better understanding of VCM system operation, I’m going to have to find a more level road and/or a dyno to test on and get a scan tool with enhanced data for this 2012 Pilot where we can accurately monitor VCM operation. Given the simplicity of how the valves are deactivated and the potential efficiency gains, my bet is that Honda will continue to rely on this system.